1.1. Technical Field
The present invention relates to an initiation system for the anionic polymerization of (meth)acrylic monomers, this system being particularly useful for initiation of anionic polymerization of methyl methacrylate in order to get syndiotactic poly(methyl methacrylate) (PMMA), for initiation of anionic polymerization of monomer acrylates where the carbon in .alpha. of the oxygen of the ester is tertiary, secondary or particularly primary (hereafter we will use the designation of tertiary, secondary or primary acrylate), for initiation of random or block copolymerization of (meth)acrylic monomers, as well as for the formation of star copolymers comprising arms formed of (meth)acrylic blocks linked to the core or node formed from a multifunctional compound (for example a deactivating agent or a monomer leading to a polymer, whether or not crosslinked).
1.2. Description of Related Art
In European Patent Application EP-A-0524054, the anionic polymerization of alkyl (meth)acrylates using an initiation system comprising a monofunctional or difunctional initiator and a ligand formed by an alkoxy alcoholate of an alkaline metal is described.
With this process, polymerization is well controlled, especially at low temperatures.
The anionic synthesis of syndiotactic PMMA or polyacrylates with controlled mass and polymolecularity required the use of sterically "encumbered" initiators to avoid secondary reactions during initiation. Since the use of pure butyllithium (n-, sec.-, tert.-) as initiator does not permit an initiation efficiency of more than 20% to be achieved, the system proposed to date generally included (1) the initiator 1,1-diphenylhexyllithium - reaction product of n-butyllithium with 1,1-diphenylethylene - and (2) an alkoxy alcoholate ligand as described in EP-A-0524054. The use of 1,1-diphenylethylene makes this process more expensive.
PMMA intrinsically, that is, when it contains nothing other than repetitive units, has a remarkable resistance to aging. Conventional initiation with 1,1-diphenylhexyllithium--the bearer of two aromatic rings--entails the presence of aromatic groups fixed at the beginning of the polymer chain. It is well known that the presence of aromatic groups diminishes the resistance to aging of polymers that contain them, which thus entails a disadvantage for the PMMA.
Th. Zundel, J. Ming Yu, L. Lestel, D. Teyssie and S. Boileau, Macromol. Symp. 88, 177-189 (1994) described trimethylsilylmethyllithium as an initiator for anionic polymerization for cyclosiloxanes and vinyl monomers. Although this article indicates that this organolithiated compound also initiates anionic polymerization of methyl methacrylate, only a few preliminary results were reported. With methyl methacrylate, there are extensive secondary reactions when the polymerization takes place at 0.degree. C. according to the process described, which entails a broad distribution of molecular weights, regardless of the solvent used. In all cases, the number average molecular weights (Wn) measured are significantly higher than the theoretical values. This probably is due to the fact that the initiation rate is low compared to the propagation rate. A substantial fraction of the initiator remains unused upon completion of polymerization. The microstructures of samples of PMMA obtained by this process were analyzed by NMR.sup.1 H. They are similar to those observed traditionally for PMMAs prepared with initiators of the organolithium type. A high level of isotactic triads is observed in toluene with lithium counter-ions. In contrast to this, the syndiotactic triads predominate when the polymerization takes place in THF at -78.degree. C. or when AlEt.sub.3 is added to the organolithium initiator in toluene. Now, PMMA with a high level of syndiotactic triads is interesting, because the polymer then has a higher glass transition temperature and thus greater heat resistance. In the prior process, PMMA could not be prepared except with maximum contents of syndiotactic triads of 67%, by working in the presence of an additive such as AlEt3 over a very long period (16 hours), in a polar medium at very low temperatures. This process is thus far from being usable in industry.